SURFACE INSTABILITIES DUE TO INTERFACIAL CHEMICAL-REACTION

When an aqueous solution of cetyltrimethylammonium bromide is contacte d with a nitrobenzene solution containing picric acid, large scale mot ion of the interface is sometimes observed, accompanied by interfacial electrical potential oscillations with a period of order 3-10 s. This behavior has been interpreted using a stability analysis of a two-pha se system (alpha, beta) in which a solute A (in phase alpha) diffuses to the interface where it reacts with solute B (from phase beta) to fo rm product P. Kinetics of the surface reaction are assumed to be infin itely fast. The stability of the system was examined with respect to s mall perturbations in the spirit of normal mode stability analysis. Bo th oscillatory and stationary regimes were identified. For the simplif ied case in which component A is insoluble in phase beta and component s B and P are insoluble in phase alpha, the presence of three diffusin g components considerably modifies the stability criteria relative to those for the diffusion of a single component across the interface. It is found that, over a narrow concentration range, an oscillatory inst ability with a period of order 1 s is predicted. This compares well wi th observed experimental results.